An ink jet recording system is a printing method in which droplets of an ink are rendered to fly and attach onto a recording medium such as paper to perform printing. By the recent innovative development of ink jet technology, an ink jet recording method has come to be used also in high-definition image recording (printing) which has hitherto been a field of photography or offset printing, and high-quality printing has been demanded not only to plain paper generally used and exclusive paper for ink jet recording (mat types and gloss types), but also to a recording medium such as printing paper.
As an ink used in ink jet recording, there is generally known an ink composition containing a coloring material acting as a coloring agent, water, an water-soluble organic compound, a surfactant and the like.
AS the coloring agent, a dye or a pigment is used. In particular, a water-soluble dye is used in a color ink in many cases for reasons of high chroma saturation of color, transparency, high solubility in water and the like. However, such a dye is generally insufficient in terms of light resistance and gas resistance, and also has a problem with water resistance because it is soluble in water. Accordingly, recorded matter recorded by the ink using the water-soluble dye has the disadvantage that it is poor in keeping quality of a recorded image. In contrast, a coloring agent insoluble in water is advantageous in water resistance. In particular, of the coloring agents insoluble in water, the pigment is a coloring material excellent in such light resistance, gas resistance and water resistance, so that the development of pigment inks taking advantage of this coloring material characteristic has been promoted. There have been proposed, for example, an aqueous pigment ink in which a pigment is dispersed with a surfactant or a polymer dispersant, an aqueous pigment ink using a self-dispersing pigment in which a water-dispersible functional group is imparted to a pigment surface, an aqueous pigment ink using fine colored particles in which a pigment is coated with a water-dispersible resin, and the like.
Such pigment inks are one in which pigment particles (including pigment-containing fine colored pigments) are stably dispersed in an ink solution. However, in the course of drying of the ink after the ink has been attached onto a recording medium such as paper, water and a volatile solvent evaporate, whereby a stable dispersion system is broken to cause easy occurrence of pigment coagulation. When the coagulated pigment precipitates on the recording medium such as paper, there is a problem that the smoothness of a surface thereof is lost, which causes a decrease in glossiness or deterioration in uniformity of a printed image to fail to obtain a high image quality image. Further, there has been a problem that a color having a hue different from an original hue comes to be seen by reflected light of the precipitated pigment (bronzing phenomenon), resulting in failure to obtain an image having a uniform desired hue. Furthermore, there have also been problems with fixing property that the pigment precipitated on the surface peels off by scratch, and problems with scratch resistance such as gloss change that when touched with a finger or the like, the pigment on the surface is scratched or broken to cause a change in glossy texture.
As for the image qualities of the ink pigment, it is proposed to restrict the particle size of a pigment dispersed in an ink (see patent document 1) and to incorporate a pigment and a pigment derivative at the same time (see patent document 2), for the purpose of improving glossiness, dispersibility and the like, and it is also proposed to incorporate fine polymer particles (see patent document 3) and to incorporate fine inorganic particles (see patent document 4), for the purpose of improving blurring and color developability. Further, there are proposed an ink containing a pigment-containing water-insoluble vinyl polymer dispersion (see patent document 5) and the like, for the purpose of improving print density in plain paper and glossiness in ink jet exclusive paper.
However, such pigment inks are also insufficient in glossiness and uniformity of printed images, and particularly have problems that the bronzing phenomenon in which a color having a hue different from an original hue comes to be seen by reflected light of the pigment coagulated on the surface of paper, fixing property of the inks and scratch resistance such as gloss change are inferior to the qualities of a water-soluble dye ink.
Patent Document 1: JP-A-2002-356637
Patent Document 2: JP-A-2002-294134
Patent Document 3: JP-A-2001-123098
Patent Document 4: JP-A-11-209671
Patent Document 5: JP-A-2005-41994